A voltage regulator module (VRM) is used to regulate a DC voltage supplied to a load, such as microprocessor. A VRM includes a power converter, such as a DC-DC converter, and may include other components such as a controller for controlling operation of the power converter. An example of a DC-DC converter is a synchronous buck converter, as shown in FIG. 1, which has minimal components, and therefore is widely used in VRM applications. In microprocessor applications, the input voltage to the VRM is typically 12VDC. The output voltage may be 5.0VDC, 3.3 VAC, or lower.
As microprocessors become more advanced, required supply voltages become lower. Supply voltages are expected to be as low as 0.5 VDC in the near future, which will require currents up to 200 A or more. Currently, the CPU of a typical personal computer operates at 3 GHz, and operating frequencies are expected to reach 10 GHz in the near future.
A consequence of the low supply voltage and high clock frequency is the high slew rate (di/dt) of the load current at power up. For example, when a microprocessor wakes from sleep mode to full operating mode, the step of the output current may be as high as 200 A, with a slew rate of 1,000 A/μs or higher. The slew rate may be over 1,000 A/μs in future designs. The voltage supplied to current microprocessors is required to be regulated within 2%, and 1% for future VRMs (“VRM 9.1 DC-DC converter design guidelines”, Intel Order Number 298646-001, January 2002). The absolute value of such voltage regulation is currently 30 mV and 10 mV for future designs.
Such tight voltage regulation is required to maintain normal operation of CMOS transistors in the microprocessor under all conditions. For instance, under worst case (high slew rate of the output current) conditions, the output voltage should not drop by more than 30 mV to avoid abnormal operation of the CPU. However, the voltage drop of VRMs based on existing designs may be so large that the output voltage regulation limit may easily be exceeded.
Various VRM topologies and control methods have been proposed in an attempt to satisfy the transient response requirements of microprocessors. However, such designs are not well-suited to the harsher dynamic requirements of next generation microprocessors.
For example, simply increasing the output capacitance can reduce the output voltage ripple, and also help maintain the output voltage during a sudden load change. However, for a single phase 1.5 VDC/25 A VRM, for instance, a design that can meet the steady state and transient voltage regulation specification typically requires at least 5,000 μF output capacitance. Such filter capacitors are bulky and expensive. It is estimated that for a VRM supplying 0.5 VDC at 100 A, the required output capacitance would be more than 10,000 μF, and should have considerably lower equivalent series inductance (ESL) and equivalent series resistance (ESR) to be effective during load transients. FIG. 2 (top curve only) shows such a relationship between the output capacitance and load current for typical prior VRMs. Although multiphase topology, which helps to reduce output capacitance, may be used for applications when the load current exceeds 20 A, the value of the capacitance is still exceedingly high at high load current.
Reducing the output inductance of a buck converter can improve its dynamic response. However, the inductance can not be reduced unbounded, otherwise the output voltage ripple will increase above acceptable limits (e.g., above 10 mV for next generation microprocessors). The increased voltage ripple will in turn reduce the room for the output voltage drop during load dynamics. In addition, a larger ripple current through the filter inductor implies a larger RMS current through the power switches, which will reduce the overall efficiency of the VRM under steady state operation. Moreover, even though the inductance can be reduced for a faster dynamic response, it is not enough to provide adequate response speed for future microprocessors if the output capacitance is required to be small to reduce cost and to satisfy size and volume constraints.
Multiphase interleaved VRM topology provides two or more power converters in parallel and shares the same output capacitors among converters. In each of the power converters (or each phase), the filter inductor can be smaller than that of a single phase VRM to achieve a faster dynamic response. The large output voltage ripple in each phase due to the small inductance can be cancelled by the ripple of other phases. The more phases are in parallel, the smaller the ripple will be, but at the expense of increased circuit cost. Multiphase topology can therefore enhance the output current capability of a VRM. However, if the output current can be provided by a single phase VRM or a VRM with fewer phases, then adopting a multiphase topology or adding extra phases in parallel solely for the purpose of reducing the ripple voltage adds considerable complexity, size, and cost. More importantly, it is very difficult for a conventionally-controlled multiphase VRM to achieve the dynamic response required by future microprocessors, without having very large output capacitance.
Current mode control has a faster dynamic response than that of conventional voltage mode control in situations where only a small perturbation such as a small load change occurs. However, its dynamic performance is not superior to that of voltage mode control when a large transient occurs. More importantly, in current mode control, the current is detected by employing a sensing resistor or a current transformer. However, for an output current of 100 A or higher, it would be impractical to use a resistor to accurately and efficiently sense the current. On the other hand, a current transformer is bulky and the sensed current must be averaged, resulting in further increases in the reaction time and drop in the output voltage when a large load step happens.
The voltage droop control method takes advantage of the upper and lower limits of the VRM output voltage to gain more room for dynamic responses. When the load current is low, the reference voltage is set to be higher than the nominal value but still within the specified upper limit. When a load step-up happens, the output voltage will drop but will have more room to drop than if it were starting from the nominal value. When the load current is high, the reference voltage is set to be low; thus when a load step-down happens, the output voltage has more room for the overshoot. However, this small room is far from being enough to handle the harsh dynamic requirements of next generation microprocessors. Moreover, the voltage droop control method also requires current sensing, which again is not very practical, as discussed above.
Operating the power converter at a very high frequency will improve the dynamic response of a VRM having a very small output capacitance. However, design of an efficient power converter operating at a very high frequency is difficult. Further, the efficiency of a power converter decreases eventually to an unacceptable or unsatisfactory level as its operating frequency increases. In general, increasing the switching frequency of a power converter solely for the purpose of improving the dynamic performance is not an optimum solution.
A stepping inductor method for fast transient response of switching converters is disclosed in U.S. Pat. No. 6,188,209, issued Feb. 13, 2001 to Poon et al. Relative to the basic buck converter, this design requires significantly more circuit components, which may be difficult and expensive to implement in a multiphase interleaved VRM, because all of the components need to be repeated for each phase. Moreover, the control circuit for load transients is analog based and the output voltage is compared to fixed hysteresis reference voltages to trigger and terminate the transient operation of the converter independently of the load current conditions. This implies that the transient circuit works the same way for a 25%, 50%, and 100% load step, for instance. Therefore, the voltage response during a load transient is not regulated and may exceed the specified limits of the output voltage during many load conditions.
A transient override circuit is proposed in U.S. Pat. No. 6,696,882, issued Feb. 24, 2004 to Markowski et al. This circuit detects the load voltage level to trigger a transient operation mode of the VRM. In transient operation mode, the power switch of a buck converter is forced to be turned on, and the synchronous power switch of the buck converter is turned off, to override the current through the output inductor. However, the circuit and control method are analog based, and, importantly, are not able to regulate the output voltage during the transient.
Peterchev et al. (“Architecture and IC implementation of a digital VRM controller”, IEEE Transactions on Power Electronics, 18 (1):356-364, 2003) relates to a digital controller for a dc-dc switch mode converter. However, the reference focuses on digital control only for normal steady state operation. Saggini et al. (“An innovative digital control architecture for low-voltage, high current dc-dc converters with tight voltage regulation”, IEEE Transactions on Power Electronics, 19 (1):210-218, 2004) addresses digital control for improving the transient response of a VRM. However, this reference teaches a variable frequency control method in combination with voltage droop control, which requires accurate sensing of the load current. U.S. Patent Publication No. 2004/015098, published Aug. 5, 2004, relates to a digital controller for a VRM; however, some of the operations carried out by this controller are effected through analog circuitry.